Attempts to develop next-generation devices having unprecedented functions by controlling structures on nanoscale have been growing rapidly in late years. Among others, creating a functional device and an ultrafast device utilizing photophysical properties is one of particularly important targets. To this end, it becomes necessary and essential to elucidate a photoexcited physical phenomenon in a local area of nanoscale, including a transient response therein. In this connection, it has been proposed to introduce a new measurement technique that combines a scanning probe microscopic technique providing an ultimate spatial resolution with a laser pulse technique providing an ultimate temporal resolution, which has already led to important achievements such as elucidations of photoexcited emission and tunneling current light emission from a single molecule and a semiconductor superlattice structure, an elucidation of a local band structure and an analysis of the point defect. See Journal of Applied Physics Vol.83, No. 7, 1 Apr. 1998, page 3453 ff.; Journal of Applied Physics Vol. 88, No. 8, 15 Oct. 2000, pages 4851; Solid State Communications Vol.6 of 107, page 281 ff., 1998; and Hyomen-Kagaku (Surface Science) 20, page 337 SHIGEKAWA, hidemi “Photoexciting STM”.
A scanning probe microscope, e.g., a scanning tunneling microscope, is an apparatus in which the probe apex having a radius of curvature in the order of angstroms is brought proximate to a surface of a specimen across a distance in the order of angstroms and a tunnel junction is formed between the probe apex and the specimen surface to measure a morphology of the surface on an atomic level from a tunneling current passing through the tunnel junction. The scanning tunneling microscope, using a piezo stage that allows scanning with a precision in the order of angstroms, is capable of obtaining a surface morphology at an ultimate spatial resolution.
On the other hand, an ultrashort laser pulse apparatus is an apparatus that is capable of producing a laser pulse having a full width of half maximum in the order of femtoseconds at an ultimate temporal resolution as short as femtoseconds.
If a specimen surface below the probe apex in the scanning tunneling microscope is irradiated with an ultrashort laser pulse to detect a tunneling current synchronized with the ultrashort laser pulse irradiation, it is then possible to measure a photoexcited physical phenomenon at an ultimate resolution both spatial and temporal, thus making it possible to derive the knowledge that is extremely important in creating a nanoscale device.
One such apparatus is a delay time modulated and time-resolved scanning probe microscope apparatus. In this apparatus, a specimen disposed in a scanning probe microscope is irradiated with a series of ultrashort laser pulse pairs for exciting the specimen while the delay time between the two laser pulses in each pulse pair is varied, and the probe signal is measured as a function of the delay time. This apparatus with the ability to measure a transient response of a nanoscale local area to a photoexcited physical phenomenon allows deriving from it the knowledge of the photoexcited physical phenomenon that is needed in creating a functional device and an ultrafast device utilizing photophysical properties. For example, the use of an ultrashort laser pulse capable of exciting a carrier in a small or nanoscale area in a specimen allows determining a lifetime of the carrier from a delay time dependency of a probe signal component that is dependent on the delay time.
Mention is now made of the construction and operation of a conventional time resolved scanning probe microscope apparatus. FIG. 9 shows such a construction of a conventional time-resolved scanning probe microscope apparatus. Here, the scanning probe microscope is illustrated as being a scanning tunneling microscope.
Referring to FIG. 9, ultrashort laser pulses 62 at a given repetition rate from a ultrashort laser pulse generator 60 are split by an interferometric delay circuit 61 into two pulses 63 and 64 having a delay time td of 1 ps (picosecond) or so. The pulses 63 and 64 are chopped by a chopper 65 with a frequency ω to be incident on an area on a specimen which lies below a probe 66 in a tunneling microscope unit. A lock-in detection device 69 is used to lock-in-detect a tunneling current signal 67 using a frequency ω (68) as a reference signal to find a difference signal Idiff between a tunneling current Iirr at the time of irradiation and a tunneling current Ibak at the time of non-irradiation. This measurement is repeated while continuously changing the delay time td between the ultrashort laser pulses 63 and 64 to determine the delay time dependency of the tunneling current component Idiff that is dependent on the delay time.
The conventional apparatus mentioned above has presented problems as described below, however.
First, the probe signal component that is dependent on the delay time td is much smaller than the component that is not dependent on the delay time td, and cannot be measured with enough precision even if the dynamic range of the lock-in device is increased to its limit.
Second, the light output intensity of the ultrashort laser light pulse generator has a long-period fluctuation due to a small change in environment (such as temperature etc.) which fluctuation, however, cannot be eliminated by the conventional technique; hence it is hard to determine the delay time dependency of the probe signal at sufficient precision.
Thus, as far as the conventional technique is concerned in which a difference in probe signal between when light is irradiated and when it is not irradiated is relied on, it has been found to be difficult to measure the delay time dependency of the probe signal component at sufficient sensitivity and precision.
Third, the probe apex in the scanning probe microscope, when irradiated with ultrafast laser pulses, is thermally elongated, while it shrinks when the laser pulses are turned off. This results in changes in the spacing between the probe apex and specimen surface and in turn in changes in the tunneling probability between them. Thus, here again there ensues the difficulty in measuring the delay time dependency of the probe signal at both high sensitivity and high precision.
Also in the prior-art apparatus, ultrashort laser pulses 63 and 64 having a delay time td have an identical wavelength as shown in FIG. 9. Thus, while such an apparatus permits measuring a phenomenon in which two energy levels participate, it cannot measure a photoexcited phenomenon in which three or more energy levels take parts.
Assuming, for example, that as shown in FIG. 10 there are excited states through energy levels 71, 72 and 73, there is a case that when a carrier upon absorbing a light energy corresponding to a difference in energy between the level 71 and the level 73 is excited from the level 71 to the level 73, the relaxation time is sought to be known of this carrier relaxing to the level 72 from the level 73. In this case, the relaxation time can be found by irradiating a specimen with a light pulse 74 having an energy corresponding to a difference in energy between the level 71 and the level 73 and a light pulse 75 having an energy corresponding to a difference in energy between the level 72 and the level 73 while continuously varying the delay time td between these pulses and determining the delay time dependency of the probe signal component. However, the prior-art apparatus cannot have two short light pulses that have different wavelength to each other.
With the view to solving the abovementioned problems, it is a first object of the present invention to provide a delay time modulated and time-resolved scanning probe microscope apparatus whereby a probe signal component that is dependent on a delay time can be measured directly at both high sensitivity and high precision with a femtosecond resolution, without the need for an increased dynamic range in a lock-in detection unit, without necessitating a probe signal value while an ultrashort laser pulse is not irradiated, with no substantial influence from a long-period fluctuation in light output intensity in an ultrashort laser pulse generator and with no substantial change in tip-specimen distance in a scanning probe microscope.
With the view to solving the abovementioned problems, it is also a second object of the present invention to provide a delay time modulated and time-resolved scanning probe microscope apparatus whereby plural ultrashort laser pulses that excites the specimen can have different wavelengths to each other, whose delay time is controlled can be varied in a manner as desired.